In an electrostatic copying machine of the dry developing and image transfer type, there is provided an apparatus for cleaning a photosensitive member in order to remove any residual toner therefrom after a toner image has been transferred, thus permitting a repeated use of the photosensitive member. A variety of cleaning apparatus are known in the art, including those utilizing a magnetic brush, a fur brush, a cleaning blade or the like.
In the electrostatic copying machine of the type described, a transfer step and a separation step are both essential. An observation of the surface of the photosensitive member after a toner image has been transferred and a transfer sheet has been separated reveals that the electric field on the photosensitive member is higher in regions corresponding to the leading and the trailing edge of the transfer sheet as compared with the electric field in the vicinity of these regions. Such phenomenon is explained as a result of the occurrence of a fringe field which occurs in a portion of the photosensitive member where a potential difference exists. Specifically, a portion of the photosensitive member on which a transfer sheet is held stationary has a different potential from another portion thereof where no transfer sheet is applied, when observed after the transfer sheet has been separated. The existence of such fringe field or peak field on the photosensitive member may cause toner present within a cleaning apparatus or around the end of the cleaning blade to be attracted onto the surface of the photosensitive member under the action of such field, to form a linear toner image. Such resulting toner image may not be cleaned by conventional cleaning apparatus, and hence may remain on the photosensitive member to be transferred to the next transfer sheet as a black streak extending in a direction perpendicular to the direction of movement of the member, thus greatly degrading the copy quality. The degree of appearance of such phenomenon greatly depends on the performance of the cleaning apparatus. It may appear under normal humidity conditions if the cleaning capability thereof is reduced. In particular, when a blade type cleaning apparatus is used and the cleaning capability of the apparatus is reduced to permit toner located on a region of a usual residual potential to be partly left on the photosensitive member and allowed to move or drift around the rear side of the blade, the rear portion of the blade edge will be contaminated, particularly when the peak electrode passes under the blade.
The contamination of the rear portion of the blade edge occurs not only when the cleaning capability is degraded, but also occurs in dependence upon the timing of releasing the blade from the photosensitive member. Specifically, when the blade is released from the photosensitive member after the latter has been completely stopped, the drift of the toner is minimized. In addition, a shift of the blade away from the photosensitive member is preferably reduced, to a value on the order of 0.1 to 0.3 mm. If a greater shift is chosen, the dispersion may cause the toner to drift around the rear portion of the blade edge, causing the occurrence of the black streak. In any event, the occurrence of the black streak is caused by the presence of a peak field on the surface of the photosensitive member, and can be prevented if the peak field is removed.
A residual potential on the photosensitive member may be eliminated by applying a corona discharge from a charger or by an irradiation with light. While the elimination of a residual potential from the photosensitive member through the corona discharge or the light irradiation is very efficient, it requires a complex arrangement, causing difficulties in the amount of space required and the resulting cost. By way of example, when a charger is used to remove the residual potential, it is necessary to provide a power supply unit which produces a high voltage. Additionally, the residual potential is reduced to zero or close to zero by utilizing a corona discharge which is either a.c. or d.c. which is of a negative and a positive polarity where the photosensitive member is charged to the positive and the negative polarity, respectively. However, depending on the environmental conditions or conditions of use, the potential cannot always be reduced to zero or close to zero after the neutralizing step, leaving the difficultiy that a uniform neutralization cannot be presumed.
When irradiation with light is used, the light intensity must be determined in accordance with the optical response of the photosensitive member. In addition, if the toner remains on the surface of the photosensitive member, the potential of the photosensitive member cannot be reduced in the regions beneath such toner. When the irradiation with light and the corona discharge are used in combination, there arises a problem that a region corresponding to the residual toner remains as a residual negative image, imposing limitations in the layout of the neutralization step within the overall process.
In a copying machine of dry type, the dispersion of toner may cause a contamination of a light emitter which is utilized to perform the light irradiation, changing the light output therefrom. Hence, the light output is generally preset to be higher than necessary. However, such choice of the light output prepresents an unnecessary power dissipation, which cannot be neglected, in particular, in a compact and high speed copying machine. In addition, the choice of a higher light irradiation accelerates the wear of the photosensitive member.